An investigation has been launched by the Ministry of Defence after former military chiefs were reported to have boasted about lobbying top officials to secure contracts for private firms in breach of Whitehall rules.

The defence secretary, Philip Hammond, insisted the retired personnel wielded "no influence" over procurement but the body that restricts the commercial activity of departing civil servants said the allegations were serious.

Among those named are Lieutenant General Sir John Kiszely, ex-head of the Defence Academy and now president of the Royal British Legion, former MoD procurement chief Lieutenant General Richard Applegate, Admiral Sir Trevor Soar, Commander-in-Chief Fleet of the Royal Navy until earlier this year, and ex-head of the army Lord Dannatt.

The newspaper said its three-month investigation exposed claims of having pushed the interests of private arms firms despite being barred from doing so and of being able to push clients' interests to the prime minister and senior officials.

Curbs on the commercial activities of senior personnel when they leave public service are set in each individual case by the independent Advisory Committee on Business Appointments, based on rules set down by government.

A spokeswoman for the committee said: "We are drawing them urgently to the attention of ministers, the head of the civil service and the permanent secretary at the Ministry of Defence for them to consider what action they might take."

The committee has "no role or powers to investigate or sanction individuals who do not abide by its advice", she said.

The MoD said it was looking into whether any rules had been broken and acting to ensure it was not possible for anyone to secure "privileged access".

It is the latest in a string of cash for access scandals to have rocked Westminster over recent years.

In a statement, Hammond said: "Equipment is procured in the interests of our armed forces and not in the interests of retired personnel. Former military officers have no influence over what MoD contracts are awarded."

An MoD spokesman said: "It is clear that former chiefs acting in a commercial capacity should not have any privileged access to the MoD and we will be putting in place measures to ensure this.

"A rigorous process is used for the approval of all equipment projects which involves non-advocate scrutiny of all major investment decisions by an investment approvals committee which provides advice to ministers and the accounting officer.

"We will be looking to see if any of these individuals have broken any rules."

All four senior generals have denied doing anything wrong, the Sunday Times reported, and said they always held the best interests of the services at heart.

Lord Dannatt, who was an adviser to David Cameron before the 2010 election, said he offered to "facilitate conversations" but rejected an £8,000 monthly fee offer.

The Sunday Times said he "candidly talked about sidestepping a ban on discussion of a £400 million contract by 'targeting' the MoD's top civil servant, with whom he went to school".

Lord Stirrup, the former chief of the general staff, who was also said to be able to call on "old friend and powerful contacts" to help in a lobbying campaign, told the Sunday Telegraph that was a "totally false picture".

He said: "I was interested because they were purporting to be developing technologies that could save people's lives. I said I could facilitate conversations with people in the military. I did not offer to lobby anyone on their behalf."